The invention relates to a process for the manufacture of ligneous material boards. The term “ligneous material boards” includes flake boards and fiber boards, especially oriented strand boards (OSB) with textured surfaces on one or both sides. One form of the process involves spreading a mat of material to be cured under pressure and heat in a press from a spreading station in several layers with a mixture of lignocellulose- and/or cellulose-containing particles, such as shavings, fibers and chips to which a binding agent is added where the fine particles from the binding agent mixture are put mainly into the middle layer of the mat of material and the coarse particles are put mainly into the covering layers of the mat of material. Such a process, from which the invention sets out, is the subject of German patent applications DE 197 18 770 A1, filed May 3, 1997, and DE 197 18 771 A1, filed May 3, 1997, both of which are hereby incorporated by reference.
The above-referenced German patent applications disclose a process that provides an already fully cured ligneous material board, coming cold from storage or still hot after leaving the hot press, with a screen impression on one or both surfaces. The screen impression is applied mainly to improve the slip and stick properties of the ligneous material board. In the disclosed process, both in cyclical multiple-stage and in single-stage presses as well as in a continuously operating press, a texturing process takes place immediately at the discharge, which can be performed selectively for the production of textured surfaces, with the result that the heat still present in the freshly pressed ligneous material board with its high plasticity can be utilized before the board is put aside (stored) for cooling.
In the manufacture of oriented strand boards (OSB), in which this embossing process is mostly used, attention is increasingly given to see that the wood shavings of the two covering layers have the optimum possible geometry for achieving maximum flexural strengths. The flake geometry in the case of conventional OSB manufacture amounts to flake lengths of 100 mm to 120 mm, flake widths of 20 mm to 50 mm, and flake thickness of 0.4 mm to 0.8 mm. This circumstance results, as desired, in the best possible flexural strengths with minimum use of material. Usually, the fines that are formed in the flake production are sifted out and either fed to the middle layer or burned. Also, the spreader heads of the molding station are constructed so that any fines content in the cover layer material tends to be spread closer to the middle of board.
The present invention relates to a further development of the process described in the German patent applications above. For it has been found that, due to this method of procedure, no fines content is found on the surfaces of the boards in modern OSB production plants. The surfaces of the boards, therefore, are very slick and despite the texturing are less slip-resistant and less securely bonded.